The introduction of frame relay in the early 1990's brought lower cost, higher bandwidth, improved reliability, and simpler management control to enterprise wide area networks (WANs) as compared to X.25 and point-to-point leased-line alternatives. Frame relay, together with single-source asynchronous transfer mode (ATM) and multiprotocol label switching (MPLS) services, still dominate the enterprise WAN market for corporate Internet traffic. Such traditional standards tend to have fixed MTU sizes which may be changed by a network administrator. A customer installs one of these networks and pays a single carrier a fee associated with the reliability and bandwidth the particular network provides. For example, a network may be advertised to provide “3 and ½ nines” (99.95%) or better reliability and have a fee based on this reliability and a cost per megabytes per second (Mbps). The present cost for such a network is almost as high as the fee paid back in 1998.
Wi-Fi is a name for wireless local area network (WLAN) based on the IEEE 802.11 set of standards and WiMax is another wireless network based on the IEEE 802.16 set of standards. WiMax supports a much larger range and higher data rates as compared to Wi-Fi. With Wi-Fi and WiMax, the MTU size changes dynamically based on the line of sight between base stations and receivers.
Path MTU discovery allows a sender of Internet Protocol (IP) packets to discover a maximum transmission unit (MTU) of packets that it may send to a given destination. According to RFC4821 Packetization Layer Path MTU Discovery document, the maximum transmission unit (MTU) is the size in bytes of the largest IP packet, including the IP header and payload, that can be transmitted on a link or a path. A link is a communication facility or medium over which nodes can communicate at the link layer, i.e., the layer immediately below IP which is either IPv4 or IPv6. A path through the network is a set of links traversed by a packet between a source node and a destination node.
If a router tries to forward a packet to an interface whose MTU is smaller than the packet size, the router has two options. The router can fragment the packet into pieces small enough to fit within the MTU or it can drop the packet. If a don't fragment (DF) bit of the IP header is set, then the router should drop the packet rather than fragment it. Standard RFC 792 defines an Internet control message protocol (ICMP) message of type 3 (destination unreachable), code 4 (fragmentation needed and DF bit set) that can be returned to the sender of the packet to alert the host that the packet was too large to be transmitted without fragmentation.